1. Field of the Invention
The present invention relates generally to the field of hammer mills of the type used for crushing mineral ores and the like and more particularly to high temperature hammer mills used for such purpose.
2. Background Discussion
Hammer mills are specially constructed machines used for breaking and crushing pieces of hard, frangible materials, such as rock and mineral ores, into smaller pieces. Typical hammer mills are constructed with one or more heavy metal plates or discs to peripheral regions of which are swingingly mounted several heavy metal weights which act as hammers. These plates, with their attached hammers, are fixed to a drive shaft which is rotatably mounted within a heavy metal housing. In operation, the discs are driven by a shaft motor at relatively high angular velocity so that pieces of material fed into the mill are impacted at high velocity by the hammers. The hammers break some of these pieces of material and drive the broken pieces, as well as other, unbroken pieces, into impact with still other pieces of the material, with housing side walls and with other hammers, thereby breaking and/or further breaking the pieces. A grating installed at the bottom of the housing enables broken pieces of material smaller than the grating openings to be discharged; larger pieces are recirculated through the mill until they are broken into sufficiently small pieces to pass through the grating openings.
A more complete description of hammer mills and their operation can, for example, be found in the Handbook of Mineral Dressing, by Arthur F. Taggart, published by John Wiley and Sons, New York, 1945, pages 4-77 through 4-87, and which is incorporated herein, by specific reference, in its entirety. As described in the referenced Handbook, primary hammer mills are generally operated at a minimum rotational speed of about 550 RPM, while hammer mills used for secondary crushing are operated at speeds as high as about 3000 RPM. To this end, hammer mills are typically driven by relatively high speed motors, which usually provide a relatively low torque, and are, therefore, normally designed to be driven to full rotational speed before any material requiring fracturing into smaller pieces is fed into the machine. After being brought up to full operating speed, hammer mills are commonly kept running even during periods when no material is being processed by the machine. In the event of an unscheduled shutdown, due to some type of malfunction, a loaded hammer mill must normally be manually unloaded before it can be restarted, since hammer mill drive motors are typically not designed to start up a loaded machine.